The present invention relates generally to a mechanism for 1:1, 1+1, and UPSR path-switched protection switching, and more particularly pertains to a fast, reliable mechanism for such path-switched protection for optical interface units in synchronous optical network (SONET) multiplexer equipment.
Protection switching apparatus and methods are well known in the prior art. In telecommunications systems where reliability is critical it is common to have a working unit that is performing the desired functions (i.e. in-service) and a redundant standby unit that is available to take over the working unit""s functions if the working unit either fails or is manually removed from service. Whenever a module failure occurs it is desirable to get a failed module off-line as quickly as possible. It is desirable to effect the necessary switching to get the failed module off-line and a good module on-line with minimum delay to prevent the loss of data. Conventional methods typically employ some type of controller monitor unit which monitors the faults present in the working module, and if a fault is detected, signals the corresponding standby module, which in turn provides the protection switching. Controller monitor units, however, suffer from excessive switching delays.
It is therefore desirable to substantially reduce the time delay between the detection of a module failure and the completion of a replacement module being brought on-line, while guaranteeing that only one unit can be on-line (e.g. working) at a time and that a failed unit can always be removed from service.
The present invention, although not limited to SONET and optical technologies, allows a working optical interface unit and a standby optical interface unit pair in a Synchronous Optical Network (SONET) or a Synchronous Digital Hierarchy (SDH) to protect each other in the event of a failure of the working unit or the failure of the span connected to the working unit if both the working unit and the standby unit are connected to different spans.
SONET (Synchronous Optical Network) is a standard for optical data transport. It defines optical signals and a synchronous frame structure for multiplexed digital traffic. It is a major contender as the physical transport layer for the next decade. SONET/SDH is an International standard which has been adopted in the United States, Europe, and in Japan. It provides necessary bandwidth for today""s networks and can grow to accommodate the networks of the future. It provides advanced fault tolerance and survivability.
The present invention is concerned with Automatic Protection Switching (APS) equipment which concerns the ability of a network element to detect a failed working unit or line and switch the service to a spare (protection) unit or line. 1+1 APS pairs a protection unit or line with each working unit or line. 1:n APS provides one protection unit or line for every n working units or lines.
Having one standby unit available to protect one working unit is called 1:1 protection. If both the working and standby units are connected to separate spans (facilities) and both send and receive nominally the same signal, then the receiving terminal can choose to take its received data from either the working or standby unit. This configuration is referred to as 1+1 protection, and is used for point-to-point linear connections between two terminals. An extension to 1+1 protection is unidirectional Path-switch rings (UPSRs) where the working unit terminates a span for one direction of the ring and the standby unit terminates the span for the other direction of the ring. Both units place the same data onto the ring and, in a fault-free network, receive the same data from the two directions of the ring. Rather than taking all of the data from one of the spans, Path-switching selects the best Path (i.e., direction of the ring) for the received data on a pertributary basis, allowing some of the tributaries to be taken from one span and some from the other. This invention covers all of these cases with the same basic set of signal lines and the same system control model.
Accordingly, it is an object of the present invention to provide a fast reliable method that allows units to be protected in single unit, span, and ring configurations in a fast and reliable manner.
It is a further object of the present invention to guarantee that only one unit can be on-line (working) at a time and that a failed unit can always be removed from service.
The present invention provides the following novel features.
1) Simple hardware interface that allows very fast 1:1, 1+1, and UPSR Path switching protection with a consistent operation model for all three types of operation.
2) Layered approach allows using a subset of the signals instead of all of the signals by relying on system software to prevent conflicts, or the full set of signals to simplify the system software.
3) With full interface, all conflicts between units are resolved in hardware in a logical manner that prevents damage to either of the two units.
4) A failed unit can always be removed from service.
5) Units respond purely to their inputs and do not need to store state information. (In other words, units do need to have state machines that keep track of previous states. This feature greatly simplifies the system design and control.)
6) UPSR Path protection switching implemented as a hardware handshake between the optical interface units, which allows each unit to keep track of only its own provisioning information. (Typically, this function is implemented on a separate unit, such as a time-slot interchange unit, by having the data from both Paths available to both units so that only one unit is in-service, or by having both units aware of each others"" provisioning.)
7) The combination of hardware to perform the real-time-critical switching and using system software to control things that aren""t as speed critical (e.g., reversion control and protection due to degraded signal conditions). The preferred implementation for degraded signal applications is to have the software on the interface units handle all protection switching control by exchanging messages between the units.
8) Switch control (except for reversion control) is localized to the two units (i.e., the working and standby unit). This feature eliminates the need for a third control unit, which reduces processing time and reduces the system hardware. More importantly, this feature allows distributed control of the system protection so that protection switching on multiple unit pairs can be performed in parallel rather than in a serial manner by a common control unit.
9) Unit failure, span failure, and unit removal are all handled through the same signal pair and simple hardware.